Internal combustion engines may use variable cam timing (VCT) to improve fuel economy and emissions performance of a vehicle. The VCT device may include a vane type cam phaser that is controlled by an electromechanically actuated spool valve. The spool valve may direct flow of a hydraulic fluid, such as oil, from one side of the vane to the other, such as from a retard side to an advance side. The VCT device may include more than one oil circuit connecting one side of the vane to the other through which the flow of a hydraulic fluid may be directed. The phaser may be oil pressure actuated, wherein the actuation of the phaser is dependent on oil pressure in the circuit. Alternatively, the phaser may be cam torque actuated wherein the actuation of the phaser is dependent on torque generated during cam actuation.
One example of a cam torque actuated VCT phaser is shown by Smith et al. in U.S. Pat. No. 8,356,583. Therein, the VCT device is configured with a hydraulically activated locking pin in an intermediate position (herein also referred to as a mid-lock position). Conventional VCT devices may include a locking pin at one end of the range of the phaser. The VCT device of Smith also utilizes two independent oil circuits, herein referred to as the phasing circuit and the detent circuit. In the mid-lock VCT phaser of Smith, a piloted valve is included in the phaser's rotor assembly and is moveable from a first position to a second position. When the piloted valve is in the first position, hydraulic fluid is blocked from flowing through the piloted valve. When the piloted valve is in the second position, hydraulic fluid is allowed to flow between a detent line from the advance chamber and a detent line from the retard chamber through the piloted valve and a common line, such that the rotor assembly is moved to and held in the intermediate phase angle position relative to the housing assembly. Detent lines communicating with the advance chamber or retard chamber are blocked when the VCT phaser is at or near the intermediate position. The spool valve has three regions of operation, namely Detent or Auto-Lock, Retard, and Advance in the specified order. Specifically, when the spool valve is commanded to the retard or advance regions, the piloted valve is in the first position, and fluid is blocked from flowing through the detent circuit lines. Additionally, fluid may flow from one side of the vane to the other via the phasing circuit lines. When the spool valve is commanded to the detent region, the piloted valve is in the second position, and fluid is free to flow from the advanced or retarded chamber, through the detent lines and the piloted valve, and into the opposite chamber through a common fluid line. Additionally, fluid is blocked from flowing through the phasing circuit lines.
However, the inventors herein have identified potential issues with such a VCT system. As one example, there may be engine conditions that are temporarily unsuitable for active phaser control, such as in the case of low oil temperature. During such conditions, there may be significant delays in cam phaser response times. In such a scenario, it may be undesirable to adjust the cam phaser position according to engine speed and load, for instance, because a new engine speed-load condition may arise as the cam phaser is still moving toward a position determined by a previous engine speed-load condition. The resulting error in cam positioning can degrade engine performance. While the error may be addressed by maintaining a particular cam phaser position or by engaging the locking pin at the mid-lock phaser position, to lock the cam phaser in place with the locking pin engaged, the duty cycle of the spool valve solenoid must change dramatically, and the spool valve must travel from either the retard or advance region to the detent region. Additionally, disengaging the locking pin takes a finite amount of time, and this must occur before phasing can resume. Thus both locking and unlocking the cam phaser may require variable amounts of time which may impact response times. As such, the prolonged response times may be undesirable if a rapid return to phasing control is required or anticipated.
In one example, the issues described above may be addressed by a method for an engine comprising: moving a spool valve to move a cam torque actuated variable cam timing phaser to a locking position; holding the phaser at the locking position without engaging a locking pin for a duration; and after the duration, further moving the spool valve to engage the locking pin. In this way, the spool valve may be commanded to the detent region, thereby directing the cam phaser to the mid-lock position with the locking pin engaged, only during selected conditions. During other conditions, the cam phaser may be directed to the mid-lock position without the locking pin engaged, and the cam phaser may be held at the mid-lock position by way of closed-loop phaser control. As a result, response times associated with engaging the locking pin and response times associated with resuming normal phasing operations may be improved.
As an example, in response to engine oil temperature being lower than a threshold, an engine controller may command a VCT cam phaser to be held at a fixed target position without engaging a locking pin while the engine oil temperature remains below the threshold. Herein, the cam phaser may be held at the intermediate mid-lock position without engaging the locking pin in anticipation of a sudden change in phaser position responsive to a change in operating conditions. The cam phaser may be held at the target position for a threshold duration. If there is no change in operating conditions allowing or necessitating a change in phaser position upon elapse of the threshold duration, the controller may command an auto-lock and the locking pin may be engaged.
In this way, a cam phaser may be held at the mid-lock position with the locking pin engaged during conditions when oil pressure may be insufficient, the cam phaser has been held at the mid-lock position without the locking pin for a threshold duration, or hardware degradation is detected, while the cam phaser position is held at the target position with the locking pin not engaged during other conditions when engine oil temperature is low and the cam phaser cannot operate under closed-loop control. By not commanding the locking pin to be engaged when operating conditions may soon be suitable to return to closed-loop control, phaser response times are improved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.